To understand the biochemical changes that
occur in the brains of individuals who are
addicted to cocaine, we have taken
advantage of nuclear medicine techniques
and targeted the dopaminergic system of
detoxified cocaine users for PET studies.
Involvement of dopaminergic systems in
reinforcement is clear, but its role in the
addictive processes is much less clear. We
must understand the process that occurs
between taking the drug because it is
pleasurable and the addictive state, during
which the drug is taken whether or not it is
pleasurable. In other words, we must
differentiate the components of the drug that
give pleasure from the components involved
in compulsive drug-taking.

What is the role of dopamine in addictive
behaviors? The dopaminergic system is
complex and functions classically
(synaptically) and tonically (baseline state),
and we must approach it from both
perspectives. Our investigations of this
system have been made in its basal state and
during a pharmacological challenge. We have
tried to determine whether there are
abnormalities in the dopamine synapse in
cocaine addicts. We assessed the integrity
of the presynaptic terminal by measuring
binding of dopamine uptake inhibitors such as
methylphenidate and [11C]-cocaine to the
dopamine transporter. Postsynaptically, we
labeled the dopamine D2 receptor. (The
dopamine D2 receptor resides both
presynaptically and postsynaptically;
however, because PET technology has
relatively poor spatial resolution and the
density of postsynaptic sites is greater than
that of the presynaptic sites, most of the
signal derives from binding of [11C]-raclopride
to the postsynaptic sites.) We also measured
the rates of glucose utilization in these same
subjects to assess metabolic changes that
might occur with prolonged cocaine use.

What have we found? Detoxified cocaine
abusers bind less [11C]-raclopride to
dopamine D2 receptors in the basal ganglia
than normal controls because they have
fewer dopamine D2 receptors. This deficit
appears to be long-lasting; it persists even as
long as 4 months after detoxification. We
were also concerned that repeated cocaine
use might lead to neurotoxicity such as that
seen after methamphetamine, thinking that if
the density of transporters were reduced,
these subjects might be at a higher risk for
Parkinson's disease. We found that
detoxified cocaine abusers have dramatic
decreases in [11C]-cocaine binding compared with controls.
However, when only the high-affinity
component of binding at the dopamine
transporter in the striatum is examined, there
are no differences compared with controls.
These measures are highly variable in
controls. We saw no degeneration of
terminals after cocaine treatment, nor was
there an increase in striatal dopamine
transporters, regardless of length of time
since the last cocaine treatment.

What is the functional significance of having
a decrement in D2 receptors? We looked to
brain glucose metabolism. Detoxified cocaine
addicts show markedly less metabolic activity
in the frontal cortex and a limited decrease in
activity in the basal ganglia. However, these
are persistent deficits. We then correlated
the regional cerebral glucose metabolism with
the availability of dopamine D2 receptors.
The strongest correlations were specific and
corresponded with the dopamine projections
to the orbitofrontal cortex and cingulate gyrus
(projections that go to the striatum). Low
densities of dopamine D2 receptors were
associated with decreased metabolism in the
orbitofrontal cortex, whereas metabolism in
other regions with dopamine D2 receptors
was relatively normal.

Previously, we showed that a massive
activation of metabolic activity of these same
brain regions was associated with very
intense craving for cocaine. Decreased
metabolic activity in the orbitofrontal cortex is
also associated with obsessive-compulsive
disorder (OCD). This correspondence raises
many interesting questions: Is the
orbitofrontal cortex involved in craving?
Does cocaine-taking by addicts represent a
compulsive behavior? If the orbitofrontal
cortex is destroyed, can behavior no longer
be self-controlled? Or will individuals with a
dysfunctional orbitofrontal cortex emit
repetitive behaviors that cannot be
terminated? How does the addict behave
under conditions that will elicit drug-taking
behavior or craving? Is the orbitofrontal
cortex involved in cue-elicited behavior?
Does the decreased metabolic activity reflect
an inability to release dopamine? And how
can changes in dopamine release be
measured?

We measured relative changes in dopamine
accumulation secondary to the occupation of
the dopamine transporters by marking
postsynaptic dopamine D2 receptors with [11C]-raclopride and then giving our subjects a
dopamine uptake inhibitor (not radiolabeled).
We used methylphenidate to inhibit uptake,
because we could not give cocaine to normal
controls. Inhibiting the uptake of dopamine
should increase the amount of dopamine in
the synapse, which should compete with the
labeled raclopride. Thus, the binding of [11C]-raclopride would be related to the
number of free dopamine D2 receptors and [11C]-raclopride occupation of the dopamine
D2 receptors premethylphenidate and
postmethylphenidate can be compared as a
measure of synaptic transmission.

The responses in both the controls and
detoxified cocaine users were quite variable.
Some of the variability was due to age, not
pharmacokinetic differences: The most
robust responses were observed in young
subjects (dopamine transmission decreases
with increasing age). As expected,
methylphenidate produced a striking change
in [11C]-raclopride binding. Interestingly, the
control subjects reported a more intense
"high" than did the detoxified cocaine users
after methylphenidate. Normal controls
self-reported more restlessness. Cocaine
abusers (3-6 weeks after last cocaine)
reported that the methylphenidate made them
crave cocaine, whereas the controls did not
report such craving.

We did not expect to see the biochemical
changes we obtained. Normal controls
showed marked reductions in striatal [11C]-raclopride binding after methylphenidate,
but the cocaine abusers did not. In fact, their
dopamine D2 binding was much lower than
controls. The self-reports of "high" or
craving obtained from the detoxified users
were very blunted compared with normal
controls, suggesting that one of the long-term
effects of repeated cocaine use may be a
state of relative dopamine dysfunction.
Considering that dopamine may impart
salience and motivation to an action, the
cocaine abuser may be much less
responsive to normal stimulation. A
downregulation in dopaminergic activity may
help explain the anhedonia reported by
cocaine users, and they may be taking
cocaine to reverse the dysphoria
(self-medication). To understand craving and
the role of dopamine, it is necessary to
understand that the craving likely occurs as
the result of the ability of dopamine to
facilitate the activation of specific brain
regions, like the orbitofrontal cortex, the
hippocampus, and the striatum. In addition, it
is the pattern of activation that leads to
craving, not just one brain region and not just
the increase in dopamine. Profound
disruptions are found in the dopamine system
of cocaine addicts, but the dopamine system
is not, by itself, responsible for craving or for
addiction.

Acknowledgment

This research was supported by National
Institute on Drug Abuse Grant No. DA-06891.